Electric utilities connect alternating current (AC) generators to AC transmission lines in one of three configurations: (1) single-terminal in which one generator is attached to one end of a transmission line; (2) two-terminal in which two generators are attached to the two ends of a single transmission line; and (3) three-terminal in which three generators are connected to the ends of a T-configured transmission line, where a line tap is used to establish the T connection.
The concept of a multi-terminal (meaning more than three terminal) transmission line is generally considered only in the case of high voltage direct current (HVDC) transmission systems, whereas it is highly desirable for reasons of economics to build AC multi-terminal transmission lines by adding taps to existing AC transmission lines Each AC generator requires protection and relays located at each AC generator are programmed to provide the ability to disconnect the generator from the transmission via circuit breakers, as needed. In the two-terminal and three-terminal AC cases, protection is complicated, since each relay can only make line measurements locally, whereas the protection schemes need to take into account such issues as where on the transmission line the fault is located. Location of the fault is estimated by each relay using fault-time line impedance and a model for transmission line per unit impedance to calculate electrical distance to the fault. Each relay has a defined protection zone, so determination of the fault location is crucial to determining which zone applies. For each zone, different time/fault current parameters apply. In the case of two-terminal systems, relays at each end can accurately measure current being fed into the fault from its own end and therefore fault distance calculations work well.
In the case of three or more terminals on AC transmission lines, this is not the case. Regardless of the fault location, there exists a situation where two of the generators are providing current into the fault from the same side. Each generator relay can only measure its own contribution to the total fault current on that side. Therefore, the calculation of fault location will not be accurate and proper coordination of protection schemes at the terminals is hindered. The problem can be solved for three-terminal systems with a variety of ad hoc rules programmed into the relays. As the number of terminals increases beyond three, the problem becomes more complex, such that practical systems are limited to three terminals. The basic issue, as in many grid protection and control issues, is lack of observability. In the case of the multi-terminal protection problem, this is a limiting factor in attaching new power sources, such as wind farms, to existing transmission lines.